Analog meters of the type having a circular dial including a scale on the periphery of the dial and a rotatable pointer located at the center of the dial are widely used throughout the world. Such meters are fairly inexpensive to manufacture and provide a clear and accurate indication of the measured information. In some applications, analog meters have been replaced by digital displays. However, digital displays are generally more expensive to manufacture and require a more sophisticated electronic processor than analog meters. Digital displays also do not allow a user to see the "rate of change" in readings at a glance.
Some analog meters use a "moving magnet" or "air core" movement to control the rotation of the indication. Such movements generally include a permanent magnet rotatably mounted on a shaft within two orthogonal electrical coils. Energizing the electrical coils produces a magnetic flux that interacts with the magnetic flux of the permanent magnet. Independently controlling the power supplied to each electrical coil shifts the resulting magnetic field, thus controlling the angular position of the permanent magnet.
Although air core movements have been available for many years, they still have a number of problems. Most air core movements use two overlapping, perpendicularly oriented electrical coils commonly referred to as "cross coils." Generally, cross coil meters include a permanent magnet mounted on a shaft that is rotatably mounted within a protective case or "bobbin." The two cross coils are formed by winding loops of wire over the exterior of the bobbin. The loops of the first or inner coil are wound around the bobbin such that half of the coil is located on each side of a centrally located shaft. The loops of the second or outer coil are wound over the top of and perpendicular to the first coil such that half of the second coil is located on each side of the shaft.
To ensure that the bobbin and shaft are not damaged or deformed, each loop of each coil should be alternately wound on one side of the shaft and then on the other side of the shaft. Alternate winding ensures that an even force is applied to both sides of the bobbin during winding. Failure to alternately place each loop of each coil at approximately the same tension often causes the case or "bobbin" to deform, bends the shaft, or induces inaccuracies in the meter as described below.
After completing winding loops of wire over the exterior of the bobbin to form the coils, the ends of the wire are generally soldered to electrical pole pieces embedded in the bobbin. In order to prevent the soldering operation from melting the structure of the bobbin, the bobbin is formed of a material that has a high heat deflection and can withstand the soldering temperatures. Unfortunately, materials that can withstand high temperatures and have good structural stability generally make poor bearing materials. Therefore, either separate bearings must be used between the shaft and bobbin or poor bearings may be formed integrally in the structure of the bobbin. Separate low friction bearings provide for more efficient and smoother movement of the shaft and pointer but may add additional cost to the meter. Integral bearings formed of a poor bearing material may reduce cost but also increase friction, thus reducing bearing efficiency. In order to overcome the increased friction, the meter must use larger coils capable of providing additional torque. Larger coils consume more power, produce more heat than smaller counterparts, and may increase the overall size of the meter.
The resistance and cross-sectional area enclosed by the coils in cross coil movements are mismatched because the outer coil is wound over the top of the inner coil. This results in the outer coil being composed of a longer length of wire and thus having greater resistance than the inner coil. Failure to wind both coils at the same tension or with the same number of loops also contributes to differences in length of the wire and thus resistance between coils. It is often necessary to add a resistor to one of the coils to balance the resistance between the coils. In addition, if the coils are not wound consistently from movement to movement, including the tension on each loop, the placement of the loops, and the number of loops in each coil, it is often necessary to individually calibrate and balance each movement. Such calibrations add a great deal of expense and complexity to the manufacturing process.
Cross coil movements are also sensitive to temperature variations. The resistance of the wire used to form the coils is a function of temperature. Because the outer coil is wound over the inner coil, there is often a temperature difference between the two coils when the movement is operating due to the heating of the coils when energized by an electrical current. The change in resistance caused by such temperature differences can throw the delicate balance of the coils off, thus contributing to meter inaccuracy.
Some of the problems with cross coil movements can be reduced by using a current driving circuit to energize the coils as opposed to a voltage driving circuit. A current driving circuit is capable of maintaining a constant current in each coil, thus eliminating the need to carefully balance the resistance of the coils. However, current driving circuits require more complex electronics and are more expensive to produce than voltage driving circuits.
The amount of torque produced by a cross coil movement for a given electrical power input is a function of the amount of copper contained within each coil and the strength and radius of the permanent magnet mounted on the shaft. Cross coil movements that produce reasonable amounts of torque over a reasonable range of movement are fairly large. The size of the cross coil movement is the major factor that prevents the size of analog instruments from being reduced. The size of the movement is especially important in meters which combine both analog and digital displays or in cluster instruments that use more than one meter movement. In such instruments, the analog movement must be as small as possible to leave room for the digital display device.
In addition to cross-coil movements, U.S. Pat. No. 5,004,976, issued to Markow et al. discloses a three coil "Y" air core movement. In Markow et al., three coil assemblies are equal angularly spaced outwardly from and around a permanent magnetic that is rotatably mounted on a shaft. The individual coil assemblies are inserted into the top or bottom of pockets spaced at equal angles about a housing that surrounds and supports the permanent magnet. The coil assemblies are energized using a series of pulse width modulated signals at a chosen frequency and duration to angularly displace the permanent magnet.
The movement disclosed in Markow et al. reduces some of the problems associated with cross coil movement designs. Specifically, each coil assembly may contain equal numbers of turns of the same size and resistance wire. However, Markow et al. spaces the electrical coil on each coil assembly radially outward from the permanent magnet. The further the coils are spaced from the magnet, the greater the power input required to produce the same amount of torque. The meter of Markow et al. is also placed within a round shield pot that prevents nesting a plurality of meters to produce shorter distances between the centers of the shafts and thus indicators.
A number of applications including vehicles such as automobiles, motorcycles and commercial trucks would benefit greatly from reduced meter size and closer spacing between the centers of the shafts of the meters. Because of the importance of drivers to keep their eyes on the road, there is a limited field of vision available for often viewed important instrumentation. Reduced instrumentation size would allow more meters and thus information to be placed within the driver's field of vision.
It would be beneficial if smaller meter movements could be designed. It would also be beneficial if the movements could be placed within containers or shield pots that could be nested together to reduce the distance between the shafts of the meters. Currently, placing moving magnet meter movements off center within a round shield pot or placing meter movements within a non-circular shield pot results in the permanent magnet and thus pointer seeking a fixed homing position when the coils are not energized. This homing phenomenon is also present during operation of the coils. As the permanent magnet mounted within the shield pot rotates, it magnetizes the shield pot. The poles of the magnetic field produced in the shield pot lag the rotating magnetic field of the and permanent magnet influencing the movement of the permanent magnet, possibly causing hysteresis errors during operation of the meter.
A goal of the present invention is to reduce the problems associated with cross coil and other moving magnet movement designs. A further goal of the invention is to reduce the size of the meter movement while increasing or maintaining its efficiency and accuracy.